Thermal compensated and tensed spring compact fiber bragg grating wavelength filter device

ABSTRACT

A thermal-compensated fiber Bragg grating wavelength filter comprises an outer low thermal expansion coefficient metallic tube and an inner high thermal expansion coefficient cylindrical coil spring jacketed by the outer metallic tube and pre-stressed beforehand. The thermal-induced length difference of the outer metallic tube and the inner spring generates a stress-relieving effect or stress-increasing effect on the pre-stressed high thermal expansion coefficient cylindrical coil spring, whereby to increase or decrease the refractive index and compensate for the thermal-induced wavelength shift of the fiber Bragg grating.

FIELD OF THE INVENTION

The present invention relates to a wavelength filter device, particularly to a precision wavelength filter device whose fiber Bragg grating is jacketed and pre-stressed by springs to achieve a thermal-compensated function.

BACKGROUND OF THE INVENTION

A fiber Bragg grating (abbreviated as FBG below) is formed by an optical fiber to function as a precision wavelength filter for optical fiber communication, which reflects the light wave according to the Bragg feedback wavelength λ_(B) determined by the predetermined Bragg grating period Λ of the fiber grating. Based on the principle of Bragg diffraction, a wavelength meeting the Bragg condition is called a Bragg feedback wavelength κ_(B); the light wave having a Bragg feedback wavelength will be reflected in a direction opposite the incident direction to a scanning device Interrogator emitting the incident light for analysis, whereby to detect whether the received wavelength changes and whereby the reflected light is split and transmitted to a communication receiving device for detecting the contents of the demodulated carrier signal of the predetermined received wavelength.

The feedback Bragg wavelength λ_(B) is expressed by Equation (1):

λ_(B)=2nΛ  (1)

wherein Λ is the period of the Bragg grating, and n the effective refractive index of the optical fiber. The Bragg feedback wavelength λ_(B) is influenced by the thermal-induced variation of the refractive index of the core material or the strain-induced variation of the period of the Bragg grating.

In the application of FBG to strain measurement, while temperature is unchanged, the strain-induced variation of the period Λ of the Bragg grating is denoted by ΔΛ. ΔΛ is substituted into Equation (1) to obtain Equation (2):

Δλ_(B)=2nΔΛ  (2)

Suppose l is the gauge length of an object that a force is to apply to and Δl is the force-induced length variation of the object. According to the definition of strain ε,

ε=Δl/l=ΔΛ/Λ  (3)

AsΔl=(ΔΛ/Λ)l=((Δλ_(B)/2n)/(λ_(B)/2n))l,

ε=Δl/l=Δλ _(B)/λ_(B)  (4)

Equation (4) is valid only under the condition that temperature is unchanged and the condition that the refractive index n is a constant. Under these conditions, Δλ_(B)/λ_(B) can be used to determine the strain of a structure the FBG is stuck to.

While temperature changes, the ratio of the variation of the Bragg feedback wavelength λ_(B) to the original Bragg feedback wavelength λ_(B) is expressed by Equation (5):

Δλ_(β)/λ_(β)=(1−P _(e))Δε+(α_(f)+ζ)ΔT  (5)

wherein P_(e) is the effective photoelasticity, α_(f) the thermal expansion coefficient, ζ the thermal-optic coefficient of the fused silica fiber, Δλ_(β) the thermal-induced variation of the Bragg feedback wavelength, Δε the thermal-induced variation of the axial strain, and ΔT the temperature variation.

In fact, temperature variation changes the density of the light-conduction core glass material of the optical fiber and thus changes the refractive index n also. Even though no force is applied to FBG, temperature variation still shifts the preset Bragg feedback wavelength. For the point-to-point optical fiber communication with a constant wavelength, the abovementioned phenomenon will cause miss of information and must be overcome. It is too expensive to maintain the communication environment at a constant temperature or to maintain the refractive index of the material of FBG unchanged. It is more rational to compensate for the thermal-induced the shift of the Bragg feedback wavelength with the physic principles and at a cost as low as possible.

There have been some conventional technologies for compensating the thermal-induced shift of the Bragg feedback wavelength λ_(B), such as the devices shown in FIG. 3 and FIG. 4 in the specification of a U.S. Pat. No. 5,042,898 “Incorporated Bragg Filter Temperature Compensated Optical Waveguide Device” disclosed by Morey, et al. and the structures shown in FIGS. 3-5 and FIGS. 6-8 in the specification of a U.S. Pat. No. 6,493,486 “Thermal compensated compact Bragg gating filter” disclosed by Chen, Finisar Corporation. The abovementioned conventional devices or structures normally use two metallic materials of different thermal expansion coefficients to form different geometric structures, such as bimetallic strips, C-shape clamps or bar-in-axis structures. Temperature change will cause different length variations in different materials. The difference of length variations further leads to axial elongation, axial contraction, lateral bending, or torsion and thus results in grating period variation ΔΛ. The intentionally-generated grating period variation ΔΛ is to compensate for the thermal-induced length variation and balance the thermal-induced variation of the original Bragg grating period ΔΛ.

The specification of the present invention uses FIG. 1 and FIG. 2 to respectively illustrate the devices of the U.S. Pat. No. 5,042,898 and the structures of the U.S. Pat. No. 6,493,486. FIG. 1 is a cross-sectional diagrams of the device of the prior art U.S. Pat. No. 5,042,898, wherein 20 a fiber optical filter device 10 denotes the optical fiber passing through the fiber filter device; 13 denotes the Bragg grating portion (same as the following “segment”); 17 denotes the input fiber segment of the fiber filter device; 21 (28) denotes a first thermal-compensated member; 22 (29) denotes a second thermal-compensated member; 23 denotes a middle recess of the first thermal-compensated member 21; 24 denotes a projection of the second thermal-compensated member 22; 25 denotes a bridge portion, which bridges the optical fiber segment 17 between the entrance connecting member 26 and the projection 24 and the Bragg grating segment 13; 26 denotes the entrance connecting member; 27 denotes the connecting member of the projection of the second thermal-compensated member; 30 denotes a preloading member; 31 denotes a spring contacts are caused to resiliently yield with the result that the forces exerted on the compensating member 22. FIG. 2 is a cross-sectional diagrams of the structure of the prior art U.S. Pat. No. 6,493,486, i.e. the structure shown in FIG. 6 of the specification of the U.S. Pat. No. 6,493,486 by Chen, wherein 80 denotes the thermal-compensated Bragg grating filter device; 82 denotes the optical fiber passing through the fiber filter device; 90 denotes a plastic coating of the optical fiber; 84 denotes an axial core of the optical fiber; 86 denotes the cladding portion of the optical fiber; 88 denotes the Bragg grating segment of the optical fiber; 92 denotes an optical fiber torsion member provides a means for applying axial torsion on optical fiber 82; 104 denotes a high thermal expansion coefficient torsion-adjusting member; 104 is sized to fit within a bore formed in one end of thermal-compensated member 106; 96 denotes a attachment point of the torsion-adjusting member 104; 98 denotes a attachment point cooperating with the attachment point 96 to adjust the distance, the distance between attachment points 96 and 98 in a distance comprising the distance of Bragg grating portion 88 plus a distance suitable to accomplish the attachment of optical fiber 82 and torsion member 92; 94 denotes the attachment point of the torsion-adjusting member 104 and the thermal-compensated member 106.

In fact, there is a proportional relationship between the refractive index and the internal stress in the optical fiber. Therefore, while temperature rises, relaxing stress or torsion can decrease the refractive index to compensate for the thermal-induced wavelength shift of the grating filter device. In FIG. 1, the spring 31 of the preloading member 30, which can bounce back under a preloaded force, is used to apply force to preload the fiber grating to generate a non-axial stress on the fiber grating. While the increase of temperature causes projection 24 of the second thermal-compensated element 22, which has a higher-thermal expansion coefficient, to extend, whereby the stress of the Bragg grating segment 13 is relaxed, and whereby the refractive index is decreased to compensate for the thermal-induced wavelength shift in the grating filter device. In FIG. 2, while temperature rises, the attachment point 96 of the torsion-adjusting member 104, which has a higher thermal expansion coefficient, relaxes the Bragg grating segment 88, whereby the torsion force applied to the FBG is decreased, and whereby the refractive index is decreased to compensate for the thermal-induced wavelength shift in the grating wavelength filter device. In other words, torsion-relief can decrease the refractive index to compensate for the thermal-induced wavelength shift in the grating wavelength filter device. The abovementioned conventional technologies fully utilize Equation (5) and make the left side of Equation (5) equal to zero, i.e. Δλ_(β)/λ_(β)=0. While there is temperature variation ΔT, the temperature variation ΔT-induced strain variation Δε in the axis of the grating is adjusted in inverse proportionality. The stress or torsion is decreased in temperature increase and increased in temperature decrease to make Δλ_(β)=0 and compensate for the thermal-induced wavelength shift in the grating filter device. There has been a tendency of reducing the volume and decrease the cost in the abovementioned mechanical structures for compensation of the temperature-induced wavelength shift. However, the selection of the central Bragg feedback wavelength needs components made of different thermal expansion coefficients, preloaded member, and FBG torsion elements, which make the volume of the thermal-compensated structure increase. In comparison with so tiny an optical fiber that has an outer diameter of only 250 μm, the thermal-compensated structure is very bulky. In the telecommunication equipments room, crowded distribution frames of optical fibers occupy much space. Therefore, FBG devices must be miniaturized so that the space of the machine room can accommodate the FBG devices while the optical fibers become so popularized that they are extended FTTH (Fiber To The Home).

SUMMARY OF THE INVENTION

While a fiber Bragg grating (FBG) is used as a wavelength filter, the normal working temperature range must be determined at first. Normally, when the fiber Bragg grating is installed at ambient temperature, the compression of FBG at the lowest working temperature must be taken in consideration and reserved to prepare for the FBG shrinkage within the elastic limit when temperature falls and the FBG elongation within the elastic limit when temperature rises. FBG that is not pre-loaded is unable to vary linearly with temperature change and likely to have strain hysteresis and lose accuracy while it works at the lowest working temperature. The preloading of FBG is usually realized by pre-stressing, pre-bending or pre-twisting in the conventional technology, and generally called FBG pre-stressing, which has a disadvantage of bulky volume, as explained in the description of the prior art. In practice, pre-stressing FBG to the predetermined operating central wavelength also reserves the compression of FBG at the lowest working temperature and implies a built-in stress or torsion in FBG. According to Equation (5) and related explanation, the built-in stress or torsion can function as a starting point of temperature compensation. The thermal-compensated function will be realized as long as is provided a pair of materials with different thermal expansion coefficients or a structure to relax FBG in temperature rising.

The present invention concentrically sleeves the fiber Bragg grating with a first spring in a loosely-jacketing method and then sleeves the first spring with a second spring in the loosely-jacketing method to form the structure of a thermal-compensated wavelength filter device. The first spring has a high thermal expansion coefficient. The second spring has a low thermal expansion coefficient, wound in a different direction with respect to the first spring. The loosely-jacketing method is a jacketing method making the workpiece have uniform gaps and applying no axial friction resistance to the optical fiber. Refer to FIG. 3 a cross-sectional diagrams schematically showing a second spring sleeving a first spring in a loosely-jacketing method and wound around the first spring in a different direction. In FIG. 3, 102 denotes an optical fiber core; 201 denotes a 125 μm outside diameter bare optical fiber; 202 denotes a 250 μm outside diameter resin protection layer; 203 denotes a 125 μm outside diameter grating segment in the optical fiber core; 204 denotes a further 250 μm outside diameter resin protection layer; 301 denotes an upper segment of a fiber with jacketed 0.9 mm outside diameter high thermal expansion coefficient cylindrical tension coil spring, wherein the 0.9 mm outside diameter spring jacketed optical fiber is adopted to match the outer diameter of the popularized coating material (such as PE) of optical fiber (the reason is also available below); 302 denotes a 0.9 mm outside diameter fiber jacketing high thermal expansion coefficient cylindrical compression coil spring; 303 denotes a lower portion of a fiber with jacketed 0.9 mm outside cylindrical tension coil spring; 304 denotes a pre-tensing front-end connecting ring or glue of optical fiber and spring; 305 denotes a pre-tensing rear-end connecting ring or glue of optical fiber and spring; 306 denotes a lower-end non-pre-tensing connecting ring or glue for the fiber and the 0.9 mm outside diameter fiber-jacketing cylindrical tension coil spring; 308 denotes a non-pre-tensing connecting ring or glue for the 0.9 mm outside diameter fiber-jacketing cylindrical tension coil spring and an upper seat ring of a second spring having a low thermal expansion coefficient and wound in a different direction; 309 denotes a non-pre-tensing connecting ring or glue for the 0.9 mm outside diameter fiber-jacketing cylindrical tension coil spring and a lower seat ring of a second spring having a low thermal expansion coefficient and wound in a different direction; 311 denotes a second spring having a low thermal expansion coefficient and wound in a different direction in a loosely-jacketing method; 312 denotes a lower segment of the 0.9 mm outside diameter fiber-jacketing cylindrical tension coil spring. The lower segment 312 of the 0.9 mm outside diameter fiber jacketing cylindrical tension coil spring includes a thermal-compensated segment having an intimate initial tension strength and a high thermal expansion coefficient. The upper segment 301 of the 0.9 mm outside diameter fiber-jacketing cylindrical tension coil spring, the 0.9 mm outside diameter fiber-jacketing cylindrical compression coil spring 302, and the lower segment 303 of the 0.9 mm outside diameter fiber-jacketing cylindrical tension coil spring are joined to form the integral 0.9 mm outside diameter fiber jacketing cylindrical coil spring structure to protect the optical fiber and the fiber grating thereinside. The abovementioned structure is based on a fiber pre-stress technology disclosed by the Inventors in a patent application “Self-Tensed and Fully Spring Jacketed Optical Fiber Sensing Structure”, which was filed on Jan. 20, 2015 and numbered 104101751. The pre-tensed technology of the abovementioned patent application is realized via placing FBG inside a combination of a cylindrical tension coil spring and a cylindrical compression coil spring to achieve an adjustable response of stress and strain within the elastic limit, whereby to reduce the bulky volume of the mechanical structure of the fiber grating wavelength filter device in the conventional technology and decrease the cost thereof. However, the present invention does not claim the “Self-Tensed and Fully Spring Jacketed Optical Fiber Sensing Structure”. The winding direction (such as rightward winding) of the second spring 311 having a low thermal expansion coefficient and wound in a different direction in a loosely-jacketing method is different from the winding direction (such as leftward winding) of the high thermal expansion coefficient first spring formed by cascading the upper segment 301 of the fiber jacketing cylindrical tension coil spring, the fiber jacketing cylindrical compression coil spring 302, and the lower segment 303 of the fiber-jacketing cylindrical tension coil spring. The different winding directions are to prevent both the coaxially-arranged springs from accidentally meshed with each other by inappropriately operation and to balance the torsion caused by different lengthy variations of the inner and outer springs in temperature change.

The present invention includes two thermal-compensated mechanisms. The first thermal-compensated mechanism is the pre-stressing mechanism of FBG. The 0.9 mm outside diameter fiber-jacketing cylindrical compression coil spring 302, which is between the upper segment 301 and lower segment 303 of the 0.9 mm outside diameter fiber-jacketing cylindrical tension coil spring, is compressed at first; the compression coil spring 302 and two ends of the optical fiber grating in the axis are secured with the connecting rings or glues 304 and 305 and then released, whereby is completed the pre-stressing of FBG. The abovementioned structure is a new elasticity-based technology as a precision strain measurement for optical progression, wherein a cylindrical tension coil spring and a cylindrical compression coil spring jointly form a spring structure, and FBG is disposed in the axis of the cylindrical compression coil spring, and wherein the spring is compressed to fix the gauge length of FBG at first, and then the compressed spring is released, whereby is completed the pre-stressing of FBG. Within the elastic region, the behavior of material obeys the Hook's law, and the modulus of elasticity is used to set the pre-stressing linearly. The spring is compressed to fix FBG placed thereinside, and the then the applied force is released to complete the pre-stressing of FBG. Further, the compressed coil spring functions like a steel shell to protect the fragile FBG thereinside. The second thermal-compensated mechanism is releasing the pre-stress or pre-torsion to achieve the thermal compensation of the structural materials having different thermal expansion coefficients. The connecting rings or glues 308 and 309 fix two ends of the second spring 311, which has a low thermal expansion coefficient and is wound in a different direction in a loosely-jacketing method, to have a fixed length. While temperature rises, the fixed length has a smaller extension because of the low thermal expansion coefficient. The smaller extension constrains the larger extension from the total length of spring combination of the fiber jacketing cylindrical compression coil spring 302 and the lower segment 312 of the fiber-jacketing cylindrical tension coil spring. Such an effect forces the lower segment 312 of the fiber jacketing cylindrical tension coil spring, which has intimate initial tension strength and a high thermal expansion coefficient, to the fiber-jacketing cylindrical compression coil spring 302 having the same outer diameter (0.9 mm) and having been pre-tensed to obtain preloading beforehand. The lower segment 312 of the cylindrical tension coil spring is thermally extended to relax the preloading, and it will be equal to decrease the refractive index to avoid wavelength variation. The operation of the present invention is based on the proportional relationship between the internal stress and the refractive index. Therefore, releasing the stress or torsion in temperature rising can decrease the refractive index to compensate for the wavelength shift caused by temperature variation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional diagrams of a device disclosed in a U.S. Pat. No. 5,042,898;

FIG. 2 is a cross-sectional diagrams of a device disclosed in a U.S. Pat. No. 6,493,486;

FIG. 3 is a cross-sectional diagrams schematically showing a low thermal expansion coefficient second spring wound in a different direction in a loosely-jacketing method according to one embodiment of the present invention; and

FIG. 4 is a cross-sectional diagrams schematically showing a thermal-compensated fiber Bragg grating filter device using a low thermal expansion coefficient loosely-jacketing tube according to one embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Below, embodiments are used to demonstrate the thermal-compensated fiber Bragg grating wavelength filter device whose optical fiber is jacketed by a low-thermal expansion coefficient tube in a loosely-jacketing technology.

The thermal-compensated fiber Bragg grating wavelength filter device whose optical fiber is jacketed by a low thermal expansion coefficient second spring in a loosely-jacketing technology and wound in a different direction shown in FIG. 3 is normally used in a moisture-free and dust-free environment. While the thermal-compensated fiber Bragg grating wavelength filter device of the present invention is to be used in a moisture- and dust-containing environment, the present invention further proposes an embodiment, wherein a low thermal expansion coefficient tube jackets the structure of the thermal-compensated fiber Bragg grating wavelength filter device in a loosely-jacketing technology, as shown in FIG. 4. FIG. 4 is a cross-sectional diagrams schematically showing a thermal-compensated fiber Bragg grating wavelength filter device jacketed with a low thermal expansion coefficient tube in a loosely-jacketing technology according to one embodiment of the present invention. The embodiment shown in FIG. 4 is different from the embodiment shown in FIG. 3 in that a low thermal expansion coefficient invar second tube 314 or a quartz tube replaces the second spring 311 having a low thermal expansion coefficient and wound in a different direction in a loosely-jacketing method. In the embodiment shown in FIG. 4, the low thermal expansion coefficient connecting rings or glues 308 and 309 secure the low thermal expansion coefficient invar second tube 314 or the quartz tube therebetween to constrain the larger extension from the total length of integral structure of the fiber jacketing cylindrical compression coil spring 302 and the lower segment 312 of the cylindrical tension coil spring thereinside. Such an effect forces the lower segment 312 of the cylindrical tension coil spring having intimate initial tension strength and a high thermal expansion coefficient to the fiber jacketing cylindrical compression coil spring 302 having the same outer diameter (0.9 mm) Thereby, the stress or torsion of FBG is released to compensate for the difference of thermal expansion coefficients of the materials. The low thermal expansion coefficient invar second tube 314 or the quartz tube can also prevent moisture and dust from invading the loosely-jacketing compression spring and the FBG concentrically disposed thereinside so that they can work normally within the elasticity region. In FIG. 4, 102 denotes an optical fiber core; 201 denotes a 125 μm outside diameter bare optical fiber; 202 denotes a 250 μm outside diameter resin protection layer; 203 denotes a 125 μm outside diameter grating segment in the optical fiber core; 204 denotes a further 250 μm outside diameter resin protection layer; 300 denotes a high thermal expansion coefficient stainless-steel cylindrical coil spring; 301 denotes an upper segment of a 0.9 mm outside diameter fiber jacketing cylindrical tension coil spring; 302 denotes a 0.9 mm outside diameter fiber-jacketing cylindrical compression coil spring; 303 denotes a lower segment of the 0.9 mm-outer diameter fiber-jacketing cylindrical tension coil spring 303 denotes a lower portion of a fiber with jacketed 0.9 mm outside diameter cylindrical tension coil spring; 304 denotes a pre-tensing front-end connecting ring or glue of optical fiber and spring; 305 denotes a pre-tensing rear-end connecting ring or glue of optical fiber and spring; 306 denotes a lower-end non-pre-tensing connecting ring or glue for the fiber and the 0.9 mm outside diameter fiber-jacketing cylindrical tension coil spring; 308 denotes a non-pre-tensing connecting ring or glue for the 0.9 mm outside diameter fiber-jacketing cylindrical tension coil spring and an upper seat ring of a outside invar tube having a low thermal expansion coefficient; 309 denotes a non-pre-tensing connecting ring or glue for the 0.9 mm outside diameter fiber-jacketing cylindrical tension coil spring and a lower seat ring of a outside invar tube having a low thermal expansion coefficient; 314 denotes the low thermal expansion coefficient loosely-jacketing metallic outside tube, such as an Invar tube or a SiO₂ tube; 312 denotes a lower segment of the 0.9 mm outside diameter fiber-jacketing cylindrical tension coil spring. The lower segment 312 of the 0.9 mm outside diameter fiber jacketing cylindrical tension coil spring includes a thermal-compensated segment having an intimate initial tension strength and a high thermal expansion coefficient.

The present invention includes two thermal-compensated mechanisms. The first thermal-compensated mechanism is the pre-stressing mechanism of FBG. The 0.9 mm outside diameter fiber-jacketing cylindrical compression coil spring 302, which is between the upper segment 301 and lower segment 303 of the 0.9 mm outside diameter fiber-jacketing cylindrical tension coil spring, is compressed at first; the compression coil spring 302 and two ends of the optical fiber grating in the axis are secured with the connecting rings or glues 304 and 305 and then released, whereby is completed the pre-stressing of FBG. Based on the relationship between engineering stress and engineering strain of FBG, FBG is pre-stressed beforehand to reserve a compression amount so that FBG can function normally within the elasticity region at the lowest working temperature. In this embodiment, to preset the gauge length of FBG is realized via pre-compressing the fiber jacketing cylindrical compression coil spring 302 and securing the pre-stress connecting rings or glues for the optical fiber and spring at the two ends of the fiber jacketing cylindrical compression coil spring 302, i.e. the pre-tensing front-end connecting ring 304 and pre-tensing rear-end connecting ring 305, shown in FIG. 4. The front-end 304 and rear-end 305 are spaced by a preset distance as the gauge length and secured to the uncompressed bare optical fiber 201 inside the compression coil spring. Then, the compressed spring is released to obtain the predetermined pre-stress. Such an operation is equivalent to pre-loading FBG of various sensor structures in the conventional technology.

Suppose that the linear relationship of the FBG wavelength filter device is maintained to a working temperature of −25° C., and calculate the maximum allowable compressive strain based on the supposition. The thermal-induced wavelength shift of FBG is 1 pm/0.1° C. (1 pm=1×10⁻¹² M). The temperature drop between the ambient temperature of 25° C. and the temperature of −25° C. is 50° C. Thus, the working wavelength is decreased by about 500 pm (=0.5 nm) In designing the compression spring, it is considered that FBG should be pre-stressed to have a wavelength shift of 0.5 nm at the ambient temperature of 25° C. to provide the pre-compression required at the temperature of −25° C. According to the elasticity relationship of the fiber Bragg grating: an elastic deformation of 1 nm requires force 80 g, 40 g (=0.04 kg, i.e. P=0.04 kg) of compressive force is applied to the compression spring. While the compressive force of the compression spring is released, the compression spring will apply 40 g of preload to FBG in the opposite direction.

The gauge length of the two ends of the fiber Bragg grating is pre-stressed by the acting force P of the compression spring compressed and then released. The relationship between the acting force P and the deflection δ of the spring is expressed by Equation (6):

δ=(8nD ³ /Gd ⁴)P  (6)

wherein d is the wire diameter of the stainless steel;

-   -   D is the average pitch diameter of the coil spring;     -   G is the lateral elasticity coefficient; and     -   n is the number of effective coils.

Set a spring index c=D/d. Thus, Equation (6) can be further expressed by Equation (7) and Equation (8):

$\begin{matrix} {\delta = {\left( {8\; {{nc}^{3}/{Gd}}} \right)P}} & {{~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~}(7)} \\ {= {\left( {8\; {{nc}^{4}/{GD}}} \right)P}} & {{~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~}(8)} \end{matrix}$

The spring index c=D/d can be used to determine the dimensions and structures of springs so as to design springs having required spring outer diameters, spring inner diameters, effective coil numbers, pre-stresses, or maximum allowable compressive stresses. The lateral elasticity coefficient G is the stress required to generate a unit shear strain, determined by the intrinsic properties of the material. The deflection of a spring is inversely proportional to the G value of the material. The spring deflection δ caused by an axial load P can be worked out from Equation (6). Suppose n=50, d=0.3 mm, D=1 mm, P=0.04 kg, and G=7.5×10³ kg/mm² (the lateral elasticity coefficient of a stainless steel). From Equation (6), the deflection caused by the load P is worked out to be δ=0.25 mm. In other words, compress the compression coil spring by 0.25 mm, apply resin to the stretch spring connecting seats near two ends and the optical fiber thereinside and cure the resin to form connecting rings, or press metal to form connecting rings, and then release the pre-compressed spiral spring, whereby is obtained the compression amount reserved for the working temperature range of 50° C. to bear the maximum allowable compressive strain while the working temperature falls to −25° C.

The second thermal-compensated mechanism is releasing the pre-stress or pre-torsion of FBG to achieve the temperature compensation of the structural materials having different thermal expansion coefficients. The connecting rings or glues 308 and 309 fix two ends of the low thermal expansion coefficient invar outside tube 314, which has a low thermal expansion coefficient and jackets the spring thereinside in a loosely-jacketing method, to have a fixed length. While temperature rises, the fixed length has a smaller extension because of the low thermal expansion coefficient. The smaller extension constrains the larger total length of extension from the spring combination of the fiber jacketing cylindrical compression coil spring 302 and the lower segment 312 of the fiber-jacketing cylindrical tension coil spring. Such an effect forces the lower segment 312 of the fiber jacketing cylindrical tension coil spring, which has intimate initial tension strength and a high thermal expansion coefficient, to the fiber-jacketing cylindrical compression coil spring 302 having the same outer diameter (0.9 mm) and having been pre-tensed to obtain preloading beforehand. The thermal extension of the lower segment 312 of the fiber-jacketing cylindrical tension coil spring can relax preloading and decrease the refractive index to avoid wavelength variation.

Both the thermal-compensated FBG structure of the embodiment shown in FIG. 4, which uses the low thermal expansion coefficient loosely-jacketing metallic outside tube, such as an Invar tube or a SiO₂ tube, and the thermal-compensated FBG structure of the embodiment shown in FIG. 3, which uses the low thermal expansion coefficient second spring wound in a different direction in a loosely-jacketing method, can realize the thermal-compensated function. Further, the low thermal expansion coefficient loosely-jacketing metallic outside tube, such as an Invar tube or a SiO₂ tube, can also prevent from moisture and dust and provide an alignment condition for the linear adjustment of the high thermal expansion coefficient first spring. The thickness of the low thermal expansion metallic tube is adjusted according to the extension amount of the integral segment formed via cascading the fiber-jacketing cylindrical compression coil spring 302 and the lower segment 312 of the fiber-jacketing cylindrical tension coil spring, or according to the difference of the thermal expansion coefficients and the fixed distance between the connecting rings or glues 308 and 309 connecting two ends on the low thermal expansion coefficient invar outside tube 314 and the stretch spring thereinside. In one embodiment, the low thermal expansion coefficient metallic outside tube is a round-holed rectangular-prismatical metallic tube disposed concentrically with the optical fiber thereinside.

In the specification and drawings of the present invention, the elements denoted by an identical numeral are regarded as functioning identically or similarly. The drawings of the specification depict the main characteristics of the present invention in simplified way, neither intended to depict all the characteristics of the present invention nor intended to present the exact proportionality or numbers of the elements. The drawings are drawn according to the spirit of the present invention to present some embodiments of the present invention. However, these embodiments are only to limit the scope of the present invention but not to limit the scope of the present invention. Based on these embodiments, the persons skilled in the art can derive many other embodiments and develop many forms of the thermal-compensated fiber Bragg wavelength filter device without departing the spirit of the present invention. For example, the present invention can be realized with the embodiments respectively using different forms of metallic tube, using different combinations of cylindrical tension coil springs with different parameters, such as different spiral angles, different spring indexes c=D/d (i.e. combinations of different average pitch diameters and different wire diameters, and different lateral elasticity coefficient G

In two embodiments described in the specification, the thermal-compensated fiber Bragg wavelength filter device uses a low thermal expansion coefficient loosely jacketing metallic outside tube (such as an Invar tube or a SiO₂ tube) or a low thermal expansion coefficient second spring wound in a different direction in a loosely-jacketing method in the structure thereof. Both embodiments can achieve the thermal-compensated function and demonstrate the structure of the present invention. However, the present invention is not limited by these embodiments and the related drawing. The persons skilled in the art should be able to modify or vary these embodiments according to the specification without departing from the spirit of the present invention. Any equivalent modification or variation according to the spirit of the present invention is to be also included within the scope of the present invention, which is based on the claims stated below. 

What is claimed is:
 1. A thermal-compensated fiber Bragg grating wavelength filter device comprising: a single mode optical fiber including a fiber Bragg grating segment; a specified length of a high thermal expansion coefficient cylindrical coil spring, jacketing the single mode optical fiber disposed thereinside, and having an integral thermal-compensated segment formed via cascading a segment of a cylindrical compression coil spring pre-tensed two ends of the fiber Bragg grating segment and a segment of a high thermal expansion coefficient cylindrical tension coil spring having intimate initial tension strength; a pre-tensing front-end connecting ring or glue of optical fiber and spring, connecting the optical fiber disposed inside and the high thermal expansion coefficient spring disposed outside; a pre-tensing rear-end connecting ring or glue of optical fiber and spring, connecting the optical fiber disposed inside and the high thermal expansion coefficient spring disposed outside; a specified length of external low thermal expansion coefficient metallic tube, loosely-jacketing the integral thermal-compensated segment of the high thermal expansion coefficient cylindrical coil spring disposed inside at two ends of the metallic tube with two non-pre-stress connecting rings or connecting glues secure the integral thermal-compensated segment of the high thermal expansion coefficient cylindrical coil spring disposed inside at two ends of the metallic tube, wherein all the elements are coaxially arranged with the inside optical fiber to form the fiber Bragg grating wavelength filter device, and wherein the thermal-compensated fiber Bragg grating wavelength filter device is characterized in that the difference between the thermal-induced length variation of the total length of the external low thermal expansion coefficient metallic tube and the thermal-induced length variation of the integral thermal-compensated segment, which is inside the metallic tube, of the high thermal expansion coefficient cylindrical coil spring generates a stress-relieving effect or stress-increasing effect on the internal pre-stressed integral thermal-compensated segment of the internal high thermal expansion coefficient cylindrical coil spring to increase or decrease the refractive index and compensate for the thermal-induced wavelength shift of the fiber Bragg filter device.
 2. The thermal-compensated fiber Bragg grating wavelength filter device of claim 1, wherein the integral segment of the high thermal expansion coefficient cylindrical coil spring for temperature compensation, which is inside the metallic tube, includes a segment of the cylindrical compression coil spring located between two ends of the pre-stressed fiber Bragg grating segment and a segment of the high thermal expansion coefficient cylindrical tension coil spring having intimate initial tension strength.
 3. The thermal-compensated fiber Bragg grating wavelength filter device of claim 1, wherein the integral segment of the high thermal expansion coefficient cylindrical coil spring for temperature compensation, which is inside the metallic tube, includes a segment of the high thermal expansion coefficient cylindrical tension coil spring having intimate initial tension strength, a segment of the cylindrical compression coil spring located between two ends of the pre-stressed fiber Bragg grating, and another segment of the high thermal expansion coefficient cylindrical tension coil spring having intimate initial tension strength.
 4. The thermal-compensated fiber Bragg grating wavelength filter device of claim 1, wherein the external low thermal expansion coefficient metallic tube loosely-jacketing the integral thermal-compensated segment of the spring is a low thermal expansion coefficient cylindrical metallic tube having a specified length and disposed coaxially with the optical fiber thereinside.
 5. The thermal-compensated fiber Bragg grating wavelength filter device of claim 1, wherein the external low thermal expansion coefficient metallic tube loosely-jacketing the integral thermal-compensated segment of the spring is a low thermal expansion coefficient round-holed rectangular-prismed metallic tube having a specified length and disposed concentrically with the optical fiber thereinside.
 6. A thermal-compensated fiber Bragg grating wavelength filter device comprising: A single mode optical fiber including a fiber Bragg grating segment; a specified length of a first high thermal expansion coefficient cylindrical coil spring, jacketing the single mode optical fiber disposed thereinside, and having an integral thermal-compensated segment formed via cascading a segment of a cylindrical compression coil spring pre-tensing two ends of the fiber Bragg grating segment and a segment of a high thermal expansion coefficient cylindrical tension coil spring having intimate initial tension strength; a pre-tensing front-end connecting ring or glue of optical fiber and spring, connecting the optical fiber disposed inside and the high thermal expansion coefficient spring disposed outside; a pre-tensing rear-end connecting ring or glue of optical fiber and spring, connecting the optical fiber disposed inside and the high thermal expansion coefficient spring disposed outside; a specified length of a second low thermal expansion coefficient cylindrical coil spring jacketing the integral thermal-compensated segment in a loosely jacketing method, and wound in a different direction, wherein two non-pre-stress connecting rings or connecting glues at two ends of the second low thermal expansion coefficient cylindrical coil spring are used to secure the integral thermal-compensated segment of the first high thermal expansion coefficient cylindrical coil spring, which is disposed inside the second coil spring, and wherein all the above mentioned elements are disposed coaxially to form the thermal-compensated fiber Bragg grating wavelength filter device, and wherein the thermal-compensated fiber Bragg grating wavelength filter device is characterized in that the difference between the thermal-induced length variation of the total length of the second high thermal expansion coefficient cylindrical coil spring and the thermal-induced length variation of the integral thermal-compensated segment of the first high thermal expansion coefficient cylindrical coil spring, which is disposed inside the second coil spring, generates a stress-relieving effect or stress-increasing effect on the pre-stressed integral thermal-compensated segment of the internal high thermal expansion coefficient cylindrical coil spring to increase or decrease the refractive index and compensate for the thermal-induced wavelength shift of the fiber Bragg filter device.
 7. The thermal-compensated fiber Bragg grating wavelength filter device of claim 6, wherein the integral thermal-compensated segment of the internal high thermal expansion coefficient cylindrical coil spring is formed via cascading a segment of the cylindrical compression coil spring located between two ends of the pre-tensed fiber Bragg grating segment and a segment of the high thermal expansion coefficient cylindrical tension coil spring having intimate initial tension strength.
 8. The thermal-compensated fiber Bragg grating wavelength filter device of claim 6, wherein the integral thermal-compensated segment of the internal high thermal expansion coefficient cylindrical coil spring is formed via cascading a segment of the high thermal expansion coefficient cylindrical tension coil spring having intimate initial tension strength, a segment of the cylindrical compression coil spring located between two ends of the pre-tensed fiber Bragg grating segment, and another segment of the high thermal expansion coefficient cylindrical tension coil spring having intimate initial tension strength.
 9. The thermal-compensated fiber Bragg grating wavelength filter device of claim 6, wherein the second low thermal expansion coefficient cylindrical coil spring jacketing the integral thermal-compensated segment in a loosely-jacketing method and wound in a different direction is a cylindrical tension coil spring having a specified length and disposed coaxially with the optical fiber thereinside.
 10. The thermal-compensated fiber Bragg grating wavelength filter device of claim 6, wherein the second low thermal expansion coefficient cylindrical coil spring jacketing the integral thermal-compensated segment in a loosely-jacketing method and wound in a different direction is a cylindrical compression coil spring having a specified length and disposed coaxially with the optical fiber thereinside. 